In the automotive field, a power transmission belt or drive belt is typically used to drive one or more accessories from the engine crankshaft. There is now a trend in the automotive industry to employ a single drive belt or so-called "serpentine" belt to drive all the accessories rather than employ several or more belts and pulleys. Thus, in a typical arrangement, a single belt can be used to drive the air conditioning compressor, air pump, power steering, alternator, fan and water pump and power brake vacuum pump from the crankshaft pulley.
In order to drive the accessories without substantial belt slip and critical belt wear, it is necessary that the belt be tensioned appropriately for the torque transmitted at each of the accessory pulleys. However, even though the belt may be initially tensioned adequately, with operation, the belt is subject to some wear and some elongation. One significant cause of serpentine drive belt failure is degradation due to heat caused by belt-slip which is usually due to insufficient tension on the belt. Thus a fixed position pulley arrangement is usually inadequate to maintain proper belt tension since any belt wear or elongation has a drastic effect on belt tension. This is particularly true of high strength belts using high modulus cords which have very little extension over their operating range. The problem of maintaining proper tension is also compounded by the long length of the serpentine belt operating on many accessories. Furthermore, the torsional vibration of the engine crankshaft together with the torque changes in the accessories and the fundamental frequency of the spring operated belt tensioner can lead to large vibrations of the transmission belt which may be critical.
A number of belt tensioners have been designed and applied in an attempt to mitigate and resolve the above problems. Belt tensioners, in general, comprise an arm with a belt engaging pulley mounted on one end thereof and with the other end pivoted to the engine block. A tensioning means maintains the tension on the arm in the direction of belt stretch so that the pulley is continuously and yieldably engaged with the belt to move therewith as the belt stretches. It is desirable that such a tensioning means maintain a continuous or constant tension on the belt. It is also desirable that the tensioner have a means for dampening oscillations or other vibrations in the belt.
Approaches that continue to apply tension to the belt with an increase in belt length and allegedly provide dampening for belt vibration are shown in Foster, U.S. Pat. Nos. 4,563,167; 4,601,683 and 4,634,408; by Burris in U.S. Pat. No. 4,536,172; and by Thomey et al in U.S. Pat. No. 4,473,362. All of the foregoing use a spiral spring in combination with hydraulic fluid vibration dampening or other vibration dampening element. Hydraulic positioning elements are employed in Kraft, U.S. Pat. No. 4,355,991 to pivot a tensioner pulley; in Spraul, U.S. Pat. No. 4,283,182 to rotate the tensioner pulley off a cam; and in Wilson, U.S. Pat. No. 4,466,803 a more involved hydraulically operated piston on a bellows is used to position the tensioner. Malloy in U.S. Pat. No. 4,571,223 discloses a rather elaborate eccentric cam and straps to vary the pulley with constant tension and Arthur, U.S. Pat. No. 4,464,146 discloses use of an elastomeric spring having lower energy density in place of a steel spiral torsion spring.
Disk springs, also known as Belleville spring washers, are employed by Kraft in U.S. Pat. No. 4,270,906 to move a cam plate on the axis of the disk springs. The linear motion of this cam plate is converted into a rotary motion of an arm carrying the belt tensioning idler pulley by interacting against another cam to rotate it by a ramp type of action. Only a portion of the axial force from the disk springs (amount available dependent on the ramp angle) is available to give torque on the arm. Large variable frictional losses are probably involved due to the relative movement of the cam surfaces. Additionally, no controlled hydraulic dampening means are disclosed and this approach requires a relatively large axial space to accommodate the disk springs and other difficulties are involved with the rather complicated cam plates.
Wilson describes in U.S. Pat. Nos. 4,411,638 and 4,525,153 a direct operating tensioner that uses Belleville spring washers in series arrangement in combination with a separate hydraulic chamber with bypass flow restriction to give a special dampening characteristic. Wilson shows the same direct operating tensioner in U.S. Pat. No. 4,696,664 and discloses the use of Belleville spring washers having a cone height to thickness ratio of 1.4 to 1.8 to obtain substantially constant force over a 100 to 50% deflection range and a cone height to thickness ratio of 1.6 to 3.0 to obtain a negative load deflection relation--spring force decreases as the spring is compressed. However, a series arrangement of Belleville spring washers having a cone to disk thickness ratio larger than about 1.4 are subject to erratic deflections when operating in the negative load deflection regime. This is due to slight differences in the load versus deflection characteristic of individual Belleville washers due to manufacturing tolerances. If one of a series of a negative load deflection characteristic Belleville washers deflect slightly more than the others, it will continue to collapse to a flat or possibly reversed cone condition while the remaining Belleville washers must adjust to obtain the required overall deflection. Other Belleville washers may subsequently deflect resulting in a highly erratic belt tensioning. Such erratic operation is accentuated by belt oscillations due to other causes.
In Binder et al U.S. Pat. No. 4,151,756 bimetallic disk springs are used to tension gear belts by moving a tensioner pulley. The objective and means involves bimetallic disks which function in a different way than from the conventional Belleville or disk springs. The bimetallic disks require changes in their temperature to vary their axial position and force. A relatively complex, large tensioning structure is shown in Binder and does not appear to employ the basic advantages of Belleville or disk springs, that is, the ability to deliver large forces with relatively small movements.
Spiral torsion springs used in many tensioners require more material and volume to generate a given torque as compared to disk or Belleville springs. Disk springs can absorb and deliver more energy per unit weight as compared to spiral torsion springs. However, the movement of disk springs is quite small compared to the spiral torsion spring. Another major requirement for a belt tensioner as applied to power transmission belts on reciprocating engines involves dampening of belt oscillations due to engine torsional vibrations and belt load variations. Some belt tensioners described in the art employ other viscous fluid and frictional dampeners as well as separate hydraulic dampeners. Such devices involve additional equipment added to the tensioner and are difficult to adjust appropriately for varying installations and operations.
A number of tensioners have been patented for tensioning toothed belts and chain drives and hydraulically dampening the belt vibrations. A typical arrangement is shown in Kawashima et al U.S. Pat. No. 4,940,447 and Kawashima et al U.S. Pat. No. 4,950,209 where a piston forced by the pulley on the belt enters one side of a chamber to pressurize oil located at the opposite end of the chamber. A check valve is used to give a substantially unidirectional dampening characteristic with the dampening away from the belt and opposed to the direction of the spring force being much larger than the opposite direction. Since the piston does not pass through the chamber, any movement of the piston into the chamber requires oil flow from the pressure chamber to a reservoir chamber which contains a substantial air volume that can be displaced or pressurized by the oil. The check valve then gives very little dampening toward the belt in the direction of the spring force allowing the oil to flow freely in one direction. The high pressure oil from the high pressure chamber to the reservoir chamber leaks through the very small clearance between the piston and its housing to permit overall movement of the piston against a helical compression spring, which tensions the belt. Such a device is subject to cavitation problems in the oil of the pressure chamber and wear of piston and housing causing dampening variations. Also, a rather elongated structure is required to give the required belt takeup. A check valve is not used in a portion of Okabe U.S. Pat. No. 4,539,001 where a two piston system is employed with a unidirectional flow connection for oil dampening. Here, a spring keeps the oil under pressure at all times with vibration causing seal wear and external leakage. Okabe U.S. Pat. No. 4,790,796 uses two springs with a two piston system and a check valve to give a pressurized chamber for unidirectional dampening. Okabe U.S. Pat. No. 4,798,563 shows the addition of a bellows shaped elastic diaphragm in the reservoir chamber to replace the free piston. Suzuki U.S. Pat. Nos. 4,881,927 and 4,874,352 shows ball-type check valve tensioners with oil chambers and external oil make up lines. Other hydraulically operated tensioners using check valves and spring biased diaphragms for maintaining oil pressure above atmospheric pressure are shown in Inoue, et al, U.S. Pat. No. 4,909,777 and U.S. Pat. No. 4,911,679. All of the above hydraulically dampening tensioners differ substantially in construction and in operation from the tensioner described in this invention.